Laser processing method

ABSTRACT

A planar object to be processed  1  comprising a hexagonal SiC substrate  12  having a front face  12   a  forming an angle corresponding to an off-angle with a c-plane is prepared. Subsequently, the object  1  is irradiated with pulse-oscillated laser light L along lines to cut  5   a   , 5   m  such that a pulse pitch becomes 10 μm to 18 μm while locating a converging point P of the laser light L within the SiC substrate  12 . Thereby, modified regions  7   a   , 7   m  to become cutting start points are formed within the SiC substrate  12  along the lines  5   a   , 5   m.

TECHNICAL FIELD

The present invention relates to a laser processing method for cutting aplanar object to be processed comprising an SiC substrate along a lineto cut.

BACKGROUND ART

Attention has been drawn to SiC (silicon carbide) as a semiconductormaterial which can manufacture power devices excellent in heatresistance, high voltage resistance, and power saving. However, SiC is amaterial which has the second highest hardness behind diamond and thusis hard to process, whereby low-speed processing or frequent bladereplacement is required when a planar object to be processed comprisingan SiC substrate is to be cut by blade dicing. Hence, a laser processingmethod has been proposed, which irradiates the object with laser light,so as to form a modified region within the SiC substrate along a line tocut, thereby cutting the object along the line from the modified regionacting as a start point (see, for example, Patent Literature 1).

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Translated International Application    Laid-Open No. 2007-514315

SUMMARY OF INVENTION Technical Problem

Meanwhile, the inventors have found that the following problem existswhen cutting a planar object comprising a hexagonal SiC substrate havinga main surface forming an angle corresponding to an off-angle with ac-plane by a laser processing method such as the one mentioned above.That is, when irradiating the object with the laser light such thatfractures are easier to extend in the thickness direction of the SiCsubstrate from the modified region in order for the fractures to reach alaser light entrance surface of the SiC substrate from the modifiedregion, fractures are also easier to extend in the c-plane directionfrom the modified region.

It is therefore an object of the present invention to provide a laserprocessing method which can cut a planar object to be processedcomprising a hexagonal SiC substrate having a main surface forming anangle corresponding to an off-angle with a c-plane, accurately along aline to cut.

Solution to Problem

The laser processing method in accordance with one aspect of the presentinvention is a laser processing method for cutting a planar object to beprocessed comprising a hexagonal SiC substrate having a main surfaceforming an angle corresponding to an off-angle with a c-plane, along aline to cut, the method comprising the step of irradiating the objectwith pulse-oscillated laser light along the line such that a pulse pitchbecomes 10 μm to 18 μm while locating a converging point of the laserlight within the SiC substrate, thereby forming a modified region tobecome a cutting start point within the SiC substrate along the line.

This laser processing method irradiates the object along the line suchthat the pulse pitch (value obtained by dividing the moving speed of theconverging point of the laser light with respect to the object by therepetition frequency of the pulsed laser light) becomes 10 μm to 18 μm.Irradiating the object with the laser light under such a condition canmake fractures extend from the modified region easier in the thicknessdirection but harder in the c-plane direction. Therefore, this laserprocessing method makes it possible to cut the planar object comprisinga hexagonal SiC substrate having a main surface forming an anglecorresponding to an off-angle with the c-plane accurately along theline. The off-angle may be 0°. This makes the main surface parallel tothe c-plane.

The laser processing method in accordance with one aspect of the presentinvention may irradiate the object with the laser light along the linesuch that the pulse pitch becomes 12 μm to 14 μm. This can makefractures extend from the modified region further easier in thethickness direction but further harder in the c-plane direction.

The laser processing method in accordance with one aspect of the presentinvention may pulse-oscillate the laser light at a pulse width of 20 nsto 100 ns or 50 ns to 60 ns. This can more securely make the fracturesextend from the modified region easier in the thickness direction butharder in the c-plane direction.

The laser processing method in accordance with one aspect of the presentinvention may cut the object along the line from the modified regionacting as a start point after forming the modified region. This canyield the object accurately cut along the line.

In the laser processing method in accordance with one aspect of thepresent invention, the modified region may include a molten processedregion.

Advantageous Effects of Invention

The present invention can cut a planar object to be processed comprisinga hexagonal SiC substrate having a main surface forming an anglecorresponding to an off-angle with a c-plane, accurately along a line tocut.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a structural diagram of a laser processing device used forforming a modified region;

FIG. 2 is a plan view of an object to be processed before laserprocessing;

FIG. 3 is a sectional view of the object taken along the line III-III ofFIG. 2;

FIG. 4 is a plan view of the object after laser processing;

FIG. 5 is a sectional view of the object taken along the line V-V ofFIG. 4;

FIG. 6 is a sectional view of the object taken along the line VI-VI ofFIG. 4;

FIG. 7 is a plan view of an object to be processed to which the laserprocessing method in accordance with an embodiment of the presentinvention is applied;

FIG. 8 is a diagram illustrating a crystal structure of the object inFIG. 7;

FIG. 9 is a set of partial sectional views of the object in FIG. 7;

FIG. 10 is a partial sectional view of the object subjected to the laserprocessing method in accordance with the embodiment of the presentinvention;

FIG. 11 is a set of partial sectional views of the object subjected tothe laser processing method in accordance with the embodiment of thepresent invention;

FIG. 12 is a set of partial sectional views of the object subjected tothe laser processing method in accordance with the embodiment of thepresent invention;

FIG. 13 is a set of partial sectional views of the object subjected tothe laser processing method in accordance with the embodiment of thepresent invention;

FIG. 14 is a diagram illustrating a photograph of a cross section of anSiC substrate cut by the laser processing method in accordance with theembodiment of the present invention;

FIG. 15 is a diagram illustrating a photograph of a cross section of theSiC substrate cut by the laser processing method in accordance with theembodiment of the present invention;

FIG. 16 is a diagram illustrating a plan view photograph of the SiCsubstrate cut by the laser processing method in accordance with theembodiment of the present invention;

FIG. 17 is a perspective view for explaining c-plane fissures occurringwithin the SiC substrate;

FIG. 18 is a diagram illustrating a photograph of a cut plane of the SiCsubstrate in which the c-plane fissure is generated;

FIG. 19 is a table illustrating relationships between pulse width and IDthreshold, HC threshold, and processing margin;

FIG. 20 is a table illustrating relationships between pulse pitch and IDthreshold, HC threshold, and processing margin;

FIG. 21 is a table illustrating results of experiments concerningprocessing margin with respect to pulse width and pulse pitch;

FIG. 22 is a table illustrating results of experiments concerningprocessing margin with respect to pulse width and pulse pitch;

FIG. 23 is a table illustrating results of experiments concerningprocessing margin with respect to pulse width and pulse pitch;

FIG. 24 is a graph illustrating relationships between pulse pitch and HCthreshold;

FIG. 25 is a graph illustrating relationships between pulse pitch and IDthreshold;

FIG. 26 is a graph illustrating relationships between pulse pitch andprocessing margin;

FIG. 27 is a table illustrating results of experiments concerningprocessing margin with respect to pulse width and pulse pitch;

FIG. 28 is a table illustrating results of experiments concerningprocessing margin with respect to pulse width and pulse pitch;

FIG. 29 is a table illustrating results of experiments concerningprocessing margin with respect to pulse width and pulse pitch;

FIG. 30 is a graph illustrating relationships between pulse pitch and HCthreshold;

FIG. 31 is a table illustrating results of experiments concerningprocessing margin with respect to HC quality in the vicinity of a laserlight entrance surface;

FIG. 32 is a table illustrating results of experiments concerningprocessing margin with respect to HC quality in the vicinity of thelaser light entrance surface;

FIG. 33 is a table illustrating results of experiments concerningprocessing margin with respect to HC quality in the vicinity of thelaser light entrance surface; and

FIG. 34 is a plan view for explaining the laser processing method inaccordance with another embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

In the following, preferred embodiments of the present invention will beexplained in detail with reference to the drawings. In the drawings, thesame or equivalent parts will be referred to with the same signs whileomitting their overlapping descriptions.

The laser processing method in accordance with an embodiment of thepresent invention irradiates a planar object to be processed with laserlight along a line to cut, so as to form a modified region within theobject along the line. Therefore, the forming of the modified regionwill firstly be explained with reference to FIGS. 1 to 6.

As illustrated in FIG. 1, a laser processing device 100 comprises alaser light source 101 for causing laser light L to oscillate in apulsating manner, a dichroic mirror 103 arranged such as to change thedirection of the optical axis (optical path) of the laser light L by90°, and a condenser lens 105 for converging the laser light L. Thelaser processing device 100 further comprises a support table 107 forsupporting an object to be processed 1 which is irradiated with thelaser light L converged by the condenser lens 105, a stage 111 formoving the support table 107, a laser light source controller 102 forregulating the laser light source 101 in order to adjust the output,pulse width, and the like of the laser light L, and a stage controller115 for regulating the movement of the stage 111.

In the laser processing device 100, the laser light L emitted from thelaser light source 101 changes the direction of its optical axis by 90°with the dichroic mirror 103 and then is converged by the condenser lens105 into the object 1 mounted on the support table 107. At the sametime, the stage 111 is shifted, so that the object 1 moves relative tothe laser light L along a line to cut 5. This forms a modified region inthe object 1 along the line 5.

As illustrated in FIG. 2, the line 5 for cutting the object 1 is settherein. The line 5 is a virtual line extending straight. When forming amodified region within the object 1, the laser light L is relativelymoved along the line 5 (i.e., in the direction of arrow A in FIG. 2)while locating a converging point P within the object 1 as illustratedin FIG. 3. This forms a modified region 7 within the object 1 along theline 5 as illustrated in FIGS. 4 to 6, whereby the modified region 7formed along the line 5 becomes a starting point region for cutting 8.

The converging point P is a position at which the laser light L isconverged. The line 5 may be curved instead of being straight or a lineactually drawn on the front face 3 of the object 1 without beingrestricted to the virtual line. The modified region 7 may be formedeither continuously or intermittently. The modified region 7 may beformed either in rows or dots, and it will be sufficient if the modifiedregion 7 is formed at least within the object 1. There are cases wherefractures are formed from the modified region 7 acting as a start point,and the fractures and modified region 7 may be exposed at outer surfaces(the front face, rear face, and outer peripheral surface) of the object1.

Here, the laser light L is absorbed in particular in the vicinity of theconverging point within the object 1 while being transmittedtherethrough, whereby the modified region 7 is formed in the object 1(internal absorption type laser processing). Therefore, the front face 3of the object 1 hardly absorbs the laser light L and thus does not melt.In the case of forming a removing part such as a hole or groove bymelting it away from the front face 3 (surface absorption type laserprocessing), the processing region gradually progresses from the frontface 3 side to the rear face side in general.

By the modified region formed in this embodiment are meant regions whosephysical characteristics such as density, refractive index, andmechanical strength have attained states different from those of theirsurroundings. Examples of the modified region include molten processedregions, crack regions, dielectric breakdown regions, refractive indexchanged regions, and their mixed regions. Other examples of the modifiedregion include areas where the density of the modified region haschanged from that of an unmodified region and areas formed with alattice defect in a material of the object (which may also collectivelybe referred to as high-density transitional regions).

The molten processed regions, refractive index changed regions, areaswhere the modified region has a density different from that of theunmodified region, or areas formed with a lattice defect may furtherincorporate a fracture (fissure or microcrack) therewithin or at aninterface between the modified and unmodified regions. The incorporatedfracture may be formed over the whole surface of the modified region orin only a part or a plurality of parts thereof.

This embodiment forms a plurality of modified spots (processing scars)along the line 5, thereby producing the modified region 7. The modifiedspots, each of which is a modified part formed by a shot of one pulse ofpulsed laser light (i.e., one pulse of laser irradiation; laser shot),gather to yield the modified region 7. Examples of the modified spotsinclude crack spots, molten processed spots, refractive index changedspots, and those in which at least one of them is mixed.

Preferably, for the modified spots, their sizes and lengths of fracturesgenerated therefrom are controlled as appropriate in view of therequired cutting accuracy, the demanded flatness of cut surfaces, thethickness, kind, and crystal orientation of the object, and the like.

The laser processing method in accordance with an embodiment of thepresent invention will now be explained in detail. As illustrated inFIG. 7, the object 1 is a wafer shaped like a disk (e.g., with adiameter of 3 inches and a thickness of 350 μm) comprising an SiCsubstrate 12. As illustrated in FIG. 8, the SiC substrate 12 has ahexagonal crystal structure with its crystal axis CA tilting by an angleθ (e.g., 4°) with respect to the thickness direction of the SiCsubstrate 12. That is, the SiC substrate 12 is a hexagonal SiC substratehaving an off-angle equal to the angle θ. As illustrated in FIG. 9, theSiC substrate 12 has a front face (main surface) 12 a and a rear face(main surface) 12 b which form the angle θ corresponding to theoff-angle with a c-plane. The SiC substrate 12 has an a-plane tilted bythe angle θ with respect to the thickness direction (dash-double-dotline in the drawing) of the SiC substrate 12 and an m-plane not tiltedwith respect to the thickness direction of the SiC substrate 12.

As illustrated in FIGS. 7 and 9, a plurality of lines to cut (firstlines to cut) 5 a extending in a direction parallel to the front face 12a and a-plane and a plurality of lines to cut (second lines to cut) 5 mextending in a direction parallel to the a- and m-planes are set likegrids (each having a size of 1 mm×1 mm, for example) in the object 1. Afunctional device is formed in each region defined by the lines 5 a, 5 mon the front face 12 a of the SiC substrate 12, while metal wiring isformed in each region defined by the lines 5 a, 5 m on the rear face 12a of the SiC substrate 12. The functional device and metal wiringconstruct a power device in each chip obtained by cutting the object 1along the lines 5 a, 5 m. The SiC substrate 12 is formed withorientation flats 6 a, 6 m in directions parallel to the lines 5 a, 5 m,respectively.

The foregoing object 1 is cut along the lines 5 a, 5 m as follows.First, as illustrated in FIG. 10, an expandable tape 23 is attached tothe object 1 so as to cover the metal wiring on the rear face 12 b ofthe SiC substrate 12. Subsequently, as illustrated in FIG. 11( a), theobject 1 is irradiated along the line 5 a with the laser light Loscillated in a pulsating manner at a pulse width of 20 ns to 100 ns(more preferably 50 ns to 60 ns) such that a pulse pitch becomes 10 μmto 18 μm (more preferably 12 μm to 14 μm), while locating the convergingpoint P of the laser light L within the SiC substrate 12. This forms amodified region (first modified region) 7 a to become a cutting startpoint within the SiC substrate 12 along the line 5 a. This modifiedregion 7 a includes a molten processed region. The pulse pitch is avalue obtained by dividing the moving speed of the converging point P ofthe laser light L with respect to the object 1 by the repetitionfrequency of the pulsed laser light L.

More specifically, the modified region 7 a is formed by locating theconverging point P of the laser light L within the SiC substrate 12,while using the front face 12 a of the SiC substrate 12 as a laser lightentrance surface, and relatively moving the converging point P along theline 5 a. For each line 5, the relative movement of the converging pointP along the line 5 a is performed a plurality of times (e.g., 8 times).During this operation, a plurality of rows (first rows; e.g., 8 rows) ofmodified regions 7 a aligning with each other in the thickness directionof the SiC substrate 12 are formed with respect to each line 5 a bychanging the distance from the front face 12 a to the position of theconverging point P at each time. Here, the modified regions 7 a areformed sequentially from the rear face 12 b side of the SiC substrate 12(i.e., in descending order of distance from the laser light entrancesurface) such that the modified region 7 a second closest to the frontface 12 a acting as the laser light entrance surface of the SiCsubstrate 12 is smaller than the modified region 7 a closest to thefront face 12 a. The size of the modified regions 7 a can be adjusted bychanging the pulse energy of the laser light L, for example.

This makes fractures generated from the modified regions 7 a extend inthe thickness direction of the SiC substrate 12 and connect with eachother. In particular, fractures extending in the thickness direction ofthe SiC substrate 12 from the modified region 7 a closest to the frontface 12 a acting as the laser light entrance surface of the SiCsubstrate 12 are made to reach the front face 12 a. These are veryimportant for accurately cutting along the line 5 a the SiC substrate 12made of a material which has the second highest hardness behind diamondand thus is hard to process.

After forming the modified regions 7 a along the lines 5 a, asillustrated in FIG. 11( b), the object 1 is irradiated along the line 5m with the laser light L oscillated in a pulsating manner at a pulsewidth of 20 ns to 100 ns (more preferably 50 ns to 60 ns) such that apulse pitch becomes 10 μm to 18 μm (more preferably 12 μm to 14 μm),while locating the converging point P of the laser light L within theSiC substrate 12. This forms a modified region (second modified region)7 m to become a cutting start point within the substrate 12 along theline 5 m. This modified region 7 m includes a molten processed region.

More specifically, the modified region 7 m is formed by locating theconverging point P of the laser light L within the SiC substrate 12,while using the front face 12 a of the SiC substrate 12 as the laserlight entrance surface, and relatively moving the converging point Palong the line Sm. For each line 5, the relative movement of theconverging point P along the line Sm is performed a plurality of times(e.g., 6 times). During this operation, a plurality of rows (second rowswhose number, e.g., 6, is smaller than that of the first rows and mayalso be 1) of modified regions 7 m aligning with each other in thethickness direction of the SiC substrate 12 are formed with respect toeach line 5 m by changing the distance from the front face 12 a to theposition of the converging point P at each time. Here, the modifiedregions 7 m are formed sequentially from the rear face 12 b side of theSiC substrate 12 (i.e., in descending order of distance from the laserlight entrance surface) such that the modified region 7 m closest to thefront face 12 a acting as the laser light entrance surface of the SiCsubstrate 12 is smaller than the modified region 7 m second closest tothe front face 12 a. The size of the modified regions 7 m can beadjusted by changing the pulse energy of the laser light L, for example.

This makes fractures generated from the modified regions 7 m extend inthe thickness direction of the SiC substrate 12 and connect with eachother. In particular, fractures extending in the thickness direction ofthe SiC substrate 12 from the modified region 7 m closest to the frontface 12 a acting as the laser light entrance surface of the SiCsubstrate 12 are made to reach the front face 12 a. These are veryimportant for accurately cutting along the line 5 m the SiC substrate 12made of a material which has the second highest hardness behind diamondand thus is hard to process.

After forming the modified regions 7 m along the lines 5 m, asillustrated in FIG. 12( a), the expandable tape 23 is expanded and,while in this state, a knife edge 41 is pressed against the rear face 12b of the SiC substrate 12 along each line 5 m through the expandabletape 23. This cuts the object 1 into bars along the lines 5 m from themodified regions 7 m acting as cutting start points. At this time, theexpandable tape 23 is in the expanded state, whereby the bars of theobject 1 are separated from each other as illustrated in FIG. 12( b).

After cutting the object 1 along the lines 5 m, while the expandabletape 23 is still in the expanded state, the knife edge 41 is pressedagainst the rear face 12 b of the SiC substrate 12 along each line 5 athrough the expandable tape 23 as illustrated in FIG. 13( a). This cutsthe object 1 into chips along the lines 5 a from the modified regions 7a acting as cutting start points. At this time, the expandable tape 23is in the expanded state, whereby the chips of the object 1 areseparated from each other as illustrated in FIG. 13( b). As in theforegoing, the object 1 is cut into chips along the lines 5 a, 5 m, soas to yield a number of power devices.

Because of the following reasons, the foregoing laser processing methodcan accurately cut along the lines 5 a, 5 m the planar object 1comprising the hexagonal SiC substrate 12 having the front face 12 aforming an angle corresponding to the off-angle with the c-plane,thereby yielding pieces of the object 1 (i.e., power devices) preciselycut along the lines 5 a, 5 m.

First, the object 1 is irradiated with the laser light L along the lines5 a, 5 m such that the pulse pitch becomes 10 μm to 18 μm. Irradiatingthe object 1 with the laser light L under such a condition can makefractures extend from the modified regions 7 a, 7 m easier in thethickness direction of the SiC substrate 12 but harder in the c-planedirection. Irradiating the object 1 with the laser light L such as toyield a pulse pitch of 12 μm to 14 μm can make fractures extend from themodified regions 7 a, 7 m further easier in the thickness direction ofthe SiC substrate 12 but further harder in the c-plane direction.

The laser light L is pulse-oscillated at a pulse width of 20 ns to 100ns. This can securely make fractures extend from the modified regions 7a, 7 m easier in the thickness direction of the SiC substrate 12 butharder in the c-plane direction. Pulse-oscillating the laser light L ata pulse width of 50 ns to 60 ns can more securely make fractures extendfrom the modified regions 7 a, 7 m easier in the thickness direction ofthe SiC substrate 12 but harder in the c-plane direction.

Along the line 5 a, the modified region 7 a second closest to the frontface 12 a acting as the laser light entrance surface of the SiCsubstrate 12 is formed relatively small. This can prevent, even when thea-plane is tilted with respect to the thickness direction of the SiCsubstrate 12, fractures generated from the modified region 7 a secondclosest to the front face 12 a from extending in the a-plane directionand reaching the front face 12 a while greatly deviating from the line 5a. Along the line 5 a, the modified region 7 a closest to the front face12 a acting as the laser light entrance surface of the SiC substrate 12is formed relatively large. This can securely make fractures reach thefront face 12 a from the modified region 7 a closest to the front face12 a but hard to extend in the thickness direction of the SiC substrate12 from the modified regions 7 a. Along the line 5 m, the modifiedregion 7 m second closest to the front face 12 a acting as the laserlight entrance surface of the SiC substrate 12 is formed relativelylarge. This, coupled with the fact that fractures are easy to extend inthe thickness direction of the SiC substrate 12 from the modifiedregions 7 m, can make fractures generated from the modified region 7 msecond closest to the front face 12 a reach the front face 12 a ornearby. Along the line 5 m, the modified region 7 a closest to the frontface 12 a acting as the laser light entrance surface of the SiCsubstrate 12 is formed relatively small. This can make fracturessecurely reach the front face 12 a from the modified region 7 m, whilepreventing the front face 12 a from being damaged. As in the foregoing,fractures can securely reach the front face 12 a from the modifiedregions 7 a, 7 m along the lines 5 a, 7 m, respectively. This effect isexhibited independently of but more remarkably in concordance withnumbers of rows and order of modified regions 7 a, 7 m which will beexplained later.

A greater number of rows of modified regions 7 a are formed along eachline 5 a than that of modified regions 7 m formed along each line 5 m.This can make fractures easier to connect in the thickness direction ofthe SiC substrate 12 between all the modified regions 7 a whilepreventing them from extending greatly in the a-plane direction from themodified regions 7 a when forming each modified region 7 a even if thea-plane is tilted with respect to the thickness direction of the SiCsubstrate 12. A smaller number of rows of modified regions 7 m areformed along each line 5 m than those of modified regions 7 a formedalong each line 5 a. This can make fractures greatly extend in thethickness direction of the SiC substrate 12 from the modified regions 7m when forming each modified region 7 m. As in the foregoing, fracturescan extend in the thickness direction of the SiC substrate 12 from themodified regions 7 a, 7 m along the lines 5 a, 5 m, respectively. Thiseffect is exhibited independently of but more remarkably in concordancewith the above-mentioned sizes by which the modified regions 7 a, 7 mare formed and the order of forming them that will be explained later.

Before forming the modified regions 7 m having a moderate condition forextending fractures in the thickness direction of the SiC substrate 12,the modified regions 7 a having a severe condition for extendingfractures in the thickness direction of the SiC substrate 12 are formed.This can prevent the modified regions 7 m from inhibiting fractures fromextending in the thickness direction of the SiC substrate 12 from themodified regions 7 a in parts where the lines 5 a, 5 m intersect whenforming the modified regions 7 a. This effect is exhibited independentlyof the above-mentioned sizes and numbers of rows by which the modifiedregions 7 a, 7 m are formed.

Further, the object 1 is cut along the lines 5 m from the modifiedregions 7 m acting as start points and then along the lines 5 a from themodified regions 7 a acting as start points. This cuts the object 1along the lines 5 m that are assumed to be relatively hard to cutbecause of a smaller number of rows of the modified regions 7 m formedand then along the modified regions 5 a that are assumed to berelatively easy to cut because of a greater number of rows of themodified regions 7 a formed. As a consequence, the force required forcutting the object 1 along the lines 5 m and the force required forcutting the object 1 along the lines 5 a can be made on a par with eachother, whereby the accuracy in cutting can further be improved alongboth of the lines 5 m, 5 a. This effect is exhibited independently ofthe above-mentioned sizes and numbers of rows by which the modifiedregions 7 a, 7 m are formed.

FIG. 14 is a diagram illustrating a photograph of a cross section of theSiC substrate 12 cut along the line 5 a by the above-mentioned laserprocessing method. FIG. 15 is a diagram illustrating a photograph of across section of the SiC substrate 12 cut along the line 5 m by theabove-mentioned laser processing method. FIG. 16 is a diagramillustrating a plan view photograph of the SiC substrate 12 cut alongthe lines 5 a, 5 m by the above-mentioned laser processing method. Here,the hexagonal SiC substrate 12 having a thickness of 350 μm with anoff-angle of 4° was prepared.

First, as illustrated in FIG. 14, 8 rows of modified regions 7 aaligning with each other in the thickness direction of the SiC substrate12 were formed for each line 5 a along the lines 5 a. The modifiedregions 7 a were formed sequentially from the rear face 12 b side of theSiC substrate 12 such that the modified region 7 a second closest to thefront face 12 a acting as the laser light entrance surface of the SiCsubstrate 12 was smaller than the modified region 7 a closest to thefront face 12 a. It is seen from FIG. 14 that the forming of themodified region 7 a second closest to the front face 12 a stopsfractures generated from the modified regions 7 a from extending. As aresult, the meandering of cut surfaces with respect to the lines 5 a wassuppressed to ±4 μm or less as illustrated in FIG. 16.

The distance from the front face 12 a to the position of the convergingpoint P is 314.5 μm, 280.0 μm, 246.0 μm, 212.0 μm, 171.5 μm, 123.5 μm,79.0 μm, and 32.0 μm sequentially from the modified region 7 a on therear face 12 b side of the SiC substrate 12. The pulse energy of thelaser light L is 25 μJ, 25 μJ, 25 μJ, 25 μJ, 20 μJ, 15 μJ, 6 μJ, and 6μJ sequentially from the modified region 7 a on the rear face 12 b sideof the SiC substrate 12.

Along the lines 5 m, as illustrated in FIG. 15, 6 rows of modifiedregions 7 m aligning with each other in the thickness direction of theSiC substrate 12 were formed for each line 5 m. The modified regions 7 mwere formed sequentially from the rear face 12 b side of the SiCsubstrate 12 such that the modified region 7 m closest to the front face12 a acting as the laser light entrance surface of the SiC substrate 12was smaller than the modified region 7 m second closest to the frontface 12 a. It is seen from FIG. 15 that the forming of the modifiedregion 7 a second closest to the front face 12 a makes fracturesgenerated from the modified regions 7 a extend to the front face 12 a ornearby. As a result, the meandering of cut surfaces with respect to thelines 5 a was suppressed to ±2 μm or less as illustrated in FIG. 16.

The distance from the front face 12 a to the position of the convergingpoint P is 315.5 μm, 264.5 μm, 213.5 μm, 155.0 μm, 95.5 μm, and 34.5 μmsequentially from the modified region 7 m on the rear face 12 b side ofthe SiC substrate 12. The pulse energy of the laser light L is 25 μJ, 25μJ, 20 μJ, 20 μJ, 15 μJ, and 7 μJ sequentially from the modified region7 m on the rear face 12 b side of the SiC substrate 12.

Relationships between fractures (hereinafter referred to as “half-cuts”)reaching the front face 12 a acting as the laser light entrance surfaceof the SiC substrate 12 from the modified regions 7 a, 7 m and fractures(hereinafter referred to as “c-plane fissures”) extending in the c-planedirection from the modified regions 7 a, 7 m will now be explained.Here, as illustrated in FIGS. 17 and 18, the explanation will focus onthe modified regions 7 a harder to generate the half-cuts but easier togenerate the c-plane fissures than the modified regions 7 m.

FIG. 19 is a table illustrating relationships between pulse width and IDthreshold, HC threshold, and processing margin. Here, while changing thepulse width at 1 ns and within the range of 10 ns to 120 ns, the IDthreshold, HC threshold, and processing margin were evaluated for eachpulse width. FIG. 20 is a table illustrating relationships between pulsepitch and ID threshold, HC threshold, and processing margin. Here, whilechanging the pulse pitch within the range of 6 μm to 22 μm, thethreshold, HC threshold, and processing margin were evaluated for eachpulse pitch.

The ID threshold is the smallest value of pulse energy of the laserlight that can generate the c-plane fissure and was evaluated excellent,good, fair, and poor in descending order (i.e., in descending order ofhardness to generate the c-plane fissure). The HC threshold is thesmallest value of pulse energy of the laser light that can generate thehalf-cut and was evaluated excellent, good, fair, and poor in ascendingorder (i.e., in descending order of easiness to generate the half-cut).The processing margin is the difference between the ID and HC thresholdsand was evaluated excellent, good, fair, and poor in descending order.The total was weighted by the ID threshold, HC threshold, and processingmargin in descending order of priority and evaluated excellent, good,fair, and poor.

It is seen from the results that, as illustrated in FIG. 19, the laserlight L is preferably pulse-oscillated at a pulse width of 20 ns to 100ns, more preferably 50 ns to 60 ns. This can promote the generation ofhalf-cuts while inhibiting the c-plane fissures from occurring. The IDthreshold, processing margin, and total at the pulse width of 10 ns wereevaluated fair but closer to poor than those at the pulse width of 20ns.

It has also been seen that, as illustrated in FIG. 20, the SiC substrate12 is preferably irradiated with the laser light L along the lines 5 a,5 m such that the pulse pitch becomes 10 μm to 18 μm, more preferably 11μm to 15 μm, further preferably 12 μm to 14 μm. This can promote thegeneration of half-cuts while inhibiting the c-plane fissures fromoccurring. Since the ID threshold is evaluated fair when the pulse pitchis 10 μm, the pulse pitch is more preferably greater than 10 μm ifgreater importance is given to the suppression of the c-plane fissuregeneration.

FIGS. 21 to 23 are tables illustrating results of experiments concerningprocessing margin with respect to pulse width and pulse pitch in thecase where the laser light L is converged at a numerical aperture of0.8. These results of experiments form grounds for the evaluationsillustrated in FIGS. 19 and 20. Conditions under which the results ofexperiments of FIGS. 21 to 23 were obtained are as follows. First, usingthe hexagonal SiC substrate 12 having an off-angle of 4° with athickness of 100 μm as a subject, the converging point P of the laserlight L was moved along the line 5 a extending in a direction parallelto the front face 12 a and a-plane. On the other hand, the laser light Lwas converged at a numerical aperture of 0.8, and the converging point Pwas located at a distance of 59 μm from the front face 12 a acting asthe laser light entrance surface of the SiC substrate 12.

Based on the foregoing conditions of experiments, the modified regions 7a and states of half-cuts and c-plane fissures were observed whilechanging the energy (pulse energy) and power of the laser light L andthe pulse pitch of the laser light L. In FIGS. 21 to 23, the pulse widthof the laser light L was 27 ns, 40 ns, and 57 ns, respectively, whilethe pulse width (repetition frequency) of the laser light L was 10 kHz,20 kHz, and 35 kHz, respectively.

In the results of experiments in FIGS. 21 to 23, ST and HC indicate thatthe half-cut occurred and not, respectively. ID indicates that thec-plane fissure occurred, while LV1 to LV3 represent scales on which thec-plane fissure occurred. LV1, LV2, and LV3 refer to the respectivecases where the span in which the c-plane fissure occurred was less than150 μm, less than 450 μm, and 450 μm or more with respect to a span of40 mm (span of 20 mm×2) when the modified region 7 a was formed alongeach of two lines 5 a. The extension of the c-plan fissure in adirection perpendicular to the line 5 a was 10 μm to 20 μm in LV1 but upto about 100 μm in LV2, LV3.

FIG. 24 is a graph illustrating relationships between pulse pitch and HCthreshold. FIG. 25 is a graph illustrating relationships between pulsepitch and ID threshold. FIG. 26 is a graph illustrating relationshipsbetween pulse pitch and processing margin. These graphs were preparedaccording to the results of experiments of FIGS. 21 to 23. Asillustrated in FIGS. 24 and 25, while both of the HC and ID thresholdsincrease as the pulse width is greater, the improvement (increase) inthe HC threshold is greater than the deterioration (increase) in the HCthreshold. This means that, as illustrated in FIG. 26, the processingmargin becomes greater as the pulse width is larger. When attention isdrawn to the pulse widths of 27 ns and 57 ns, for example, at the pulsepitch of 12 μm, the HC threshold deteriorates (increases) by 2 μJ from15 μA to 17 μA while the ID threshold improves (increases) by 12 μJ from17 μJ to 29 μJ. The processing margin is seen to improve greatly withinthe range of pulse pitch of 10 μm to 16 μm at the pulse width of 40 nsover that at the pulse width of 27 ns. The processing margin is seen toimprove greatly within the range of pulse pitch of 6 μm to 20 μm at thepulse width of 57 ns over that at the pulse width of 27 ns.

FIGS. 27 to 29 are tables illustrating results of experiments concerningprocessing margin with respect to pulse width and pulse pitch in thecase where the laser light L is converged at a numerical aperture of0.6. These results of experiments form grounds for the evaluationsillustrated in FIGS. 19 and 20. Conditions under which the results ofexperiments of FIGS. 27 to 29 were obtained are as follows. First, usingthe hexagonal SiC substrate 12 having a thickness of 350 μm with thefront face 12 a forming an angle corresponding to the off-angle with thec-plane as a subject, the converging point P of the laser light L wasmoved along the line 5 a extending in a direction parallel to the frontface 12 a and a-plane. On the other hand, the laser light L wasconverged at a numerical aperture of 0.6, and the converging point P waslocated at a distance of 50 μm from the front face 12 a acting as thelaser light entrance surface of the SiC substrate 12.

Based on the foregoing conditions of experiments, the modified regions 7a and states of half-cuts and c-plane fissures were observed whilechanging the energy (pulse energy) and power of the laser light L andthe pulse pitch of the laser light L. In FIGS. 27 to 29, the pulse widthof the laser light L was 27 ns, 40 ns, and 57 ns, respectively, whilethe pulse width (repetition frequency) of the laser light L was 10 kHz,20 kHz, and 35 kHz, respectively.

In the results of experiments in FIGS. 27 to 29, ST and HC indicate thatthe half-cut occurred and not, respectively. ID indicates that thec-plane fissure occurred, while LV1 to LV3 represent scales on which thec-plane fissure occurred. The standards for LV1 to LV3 are the same asthose for the above-mentioned results of experiments of FIGS. 21 to 23.Further, OD indicates that, when the energy of the laser light L wasmade greater, the modified region 7 a also increased, thereby causingfractures to run wild and reach the front face 12 a of the SiC substrate12 while deviating much from the line 5 a. In this case, the c-planefissure was not evaluated. At the pulse widths of 40 ns and 57 ns,however, the c-plane fissure did not occur on a large scale at the pulsepitch of 12 μm or greater.

FIG. 30 is a graph illustrating relationships between pulse pitch and HCthreshold. This graph was prepared according to the results ofexperiments of FIGS. 27 to 29. As illustrated in FIG. 30, the HCthreshold was harder to occur by about 2 μJ to 4 μJ at the pulse widthof 57 ns as compared with that at the pulse width of 40 ns. Aberrationsare less influential on the converging point P of the laser light at thenumerical aperture of 0.6 than at the above-mentioned numerical apertureof 0.8, whereby about the same HC threshold was obtained at the pulsewidths of 57 ns and 40 ns. Therefore, it can be said that, even when thepulse width is large (up to at least 60 ns), the HC threshold will notdeteriorate if the aberrations are corrected.

Results of experiments concerning processing margin with respect to HCquality in the vicinity of the front face 12 a acting as the laser lightentrance surface of the SiC substrate 12 will now be explained.Conditions under which the results of experiments of FIGS. 31 to 33 wereobtained are as follows. First, using the hexagonal SiC substrate 12having an off-angle of 4° with a thickness of 100 μm as a subject, theconverging point P of the laser light L was moved along the line 5 aextending in a direction parallel to the front face 12 a and a-plane. Onthe other hand, the laser light L was converged at a numerical apertureof 0.8.

First, in the results of experiments in FIG. 31, under the irradiationwith the laser light L at each of pulse widths of 27 ns, 40 ns, 50 ns,and 57 ns, respective energies (pulse energies) generating a half-cutand not at a converging point position of 40.6 μm were used, and thestate of half-cuts was observed while changing the converging pointposition within the range of 25.3 μm to 40.6 μm. The pulse pitch of thelaser light L was fixed at 14 μm. Here, the converging point position isthe distance from the front face 12 a to the position of the convergingpoint P. As a result, the quality of half-cuts hardly deteriorateddepending on the pulse width, so that high-quality half-cuts (incurringless meandering with respect to lines to cut) were generated at thepulse width of 27 ns to 57 ns. The processing margin increased as thepulse width was greater. When the pulse width was small, branching andfissures (OD) were likely to occur in a part of half-cuts.

In the results of experiments in FIG. 32, under the irradiation with thelaser light L at each of pulse widths of 27 ns, 40 ns, 50 ns, and 57 ns,the state of half-cuts was observed while changing the pulse energywithin the range of 7 μJ to 12 μJ. The pulse pitch of the laser light Lwas fixed at 14 μm, while the converging point position was fixed at34.5 μm. As a result, the HC threshold hardly changed depending on thepulse width. Half-cuts having about the same quality were generated atthe same pulse energy.

In the results of experiments in FIG. 33, under the irradiation with thelaser light L at each of pulse pitches of 10 μm, 12 μm, 14 μm, 16 μm,and 18 μm, the state of half-cuts was observed while changing the pulseenergy within the range of 7 μJ to 12 μJ. The pulse width of the laserlight L was fixed at 57 ns, while the converging point position wasfixed at 34.5 μm. As a result, the quality of half-cuts hardly changeddepending on the pulse width. At the converging point position of 34.5μm, half cuts having about the same quality was generated by the samepulse energy.

Another laser processing method for suppressing c-plane fissures willnow be explained. First, a planar object to be processed 1 comprising ahexagonal SiC substrate 12 having a front face 12 a forming an anglecorresponding to an off-angle with the c-plane is prepared, and lines tocut 5 a, 5 m are set. Subsequently, as illustrated in FIG. 34( a), theobject 1 is irradiated with laser light L along each of two preliminarylines 5 p set on both sides of the line 5 a (5 m), while locating theconverging point P of the laser light L within the SiC substrate 12.This forms preliminary modified regions 7 p within the SiC substrate 12along each preliminary line 5 p. The preliminary modified regions 7 pinclude molten processed regions.

The preliminary lines 5 p are lines positioned on both sides of the line5 a (5 m) within a plane parallel to the front face 12 a while extendingin a direction parallel to the line 5 a (5 m). In the case where afunctional device is formed in each region defined by the lines 5 a, 5 mon the front face 12 a of the SiC substrate 12, the preliminary lines 5p are preferably set within regions between adjacent functional deviceswhen seen in the thickness direction of the SiC substrate 12.

When irradiating the object 1 with the laser light L along eachpreliminary line 5 p, fractures are made harder to occur in the SiCsubstrate 12 from the preliminary modified regions 7 p than from themodified regions 7 a (7 m) to become cutting start points. By reducingthe pulse energy, pulse pitch, pulse width, and the like of the laserlight L, the preliminary modified regions 7 p can be made harder togenerate fractures in the SiC substrate 12 than do the modified regions7 a (7 m) to become cutting start points.

After forming the preliminary modified regions 7 p along the preliminarylines 5 p, the object 1 is irradiated with the laser light L along theline 5 a (5 m) while locating the converging point P of the laser lightL within the SiC substrate 12. This forms the modified regions 7 a (7 m)to become cutting start points within the SiC substrate 12 along theline 5 a (5 m). The modified regions 7 a (7 m) include molten processedregions. After forming the modified regions 7 a (7 m) along the line 5 a(5 m), the object 1 is cut along the line 5 a (5 m) from the modifiedregions 7 a (7 m) acting as start points.

Because of the following reasons, the foregoing laser processing methodcan accurately cut along the lines 5 a, 5 m the planar object 1comprising the hexagonal SiC substrate 12 having the front face 12 aforming an angle corresponding to the off-angle with the c-plane,thereby yielding pieces of the object 1 (i.e., power devices) preciselycut along the lines 5 a, 5 m.

When forming the modified regions 7 a (7 m) within the SiC substrate 12along the line 5 a (5 m), the preliminary modified regions 7 p have beenformed within the SiC substrate 12 along each preliminary line 5 p. Thepreliminary lines 5 p are located on both sides of the line 5 a (5 m)within a plane parallel to the front face 12 a while extending in adirection parallel to the line 5 a (5 m). Therefore, the preliminarymodified regions 7 p inhibit fractures (c-plane fissures), if any, fromextending in the c-plane direction from the modified region 7 a (7 m) asillustrated in FIG. 34( a) than in the case without the preliminarymodified regions 7 p as illustrated in FIG. 34( b). This makes itpossible to irradiate the object 1 with laser light so as to makefractures easier to extend in the thickness direction of the SiCsubstrate 12 from the modified region 7 a (7 m) regardless of whether itbecomes easier for fractures to extend in the c-plane direction from themodified region 7 a (7 m) or not. The preliminary modified regions 7 p,which are not required to function as cutting start points (i.e., topromote the extension of fractures in the thickness direction of the SiCsubstrate 12 from the preliminary modified regions 7 p), are formed bysuch irradiation with the laser light L that fractures are harder tooccur in the SiC substrate 12, whereby fractures can easily be inhibitedfrom extending in the c-plane direction from the preliminary modifiedregions 7 p when forming the latter. Therefore, the planar objectcomprising the hexagonal SiC substrate 12 having a main surface formingan angle corresponding to the off-angle with the c-plane can accuratelybe cut along the line 5 a (5 m).

When locating the converging point P of the laser light L at apredetermined distance from the front face 12 a acting as the laserlight entrance surface of the SiC substrate 12 at the time of formingthe modified region 7 a (7 m), it will be preferred if the convergingpoint P of the laser light L is placed at the same distance from thefront face 12 a at the time of forming the preliminary modified region 7p. This can more securely inhibit fractures from extending in thec-plane direction from the modified region 7 a (7 m).

Forming the preliminary modified regions 7 p within the SiC substrate 12along each preliminary line 5 p and the modified regions 7 a (7 m)within the SiC substrate 12 along the line 5 a (5 m) set between thepreliminary lines 5 p at the same time also allows the preliminarymodified regions 7 p to inhibit c-plane fissures from extending. In thiscase, it is preferable to form the preliminary modified regions 7 palong the preliminary lines 5 p prior to the modified regions 7 a (7 m)along the lines 5 a (5 m).

INDUSTRIAL APPLICABILITY

The present invention can cut a planar object to be processed comprisinga hexagonal SiC substrate having a main surface forming an anglecorresponding to an off-angle with a c-plane, accurately along a line tocut.

REFERENCE SIGNS LIST

1 . . . object to be processed; 5 a, 5 m . . . line to cut; 5 p . . .preliminary line; 7 a, 7 m . . . modified region; 7 p . . . preliminarymodified region; 12 . . . SiC substrate; 12 a . . . front face (mainsurface); 12 b . . . rear face (main surface); L . . . laser light; P .. . converging point

1. A laser processing method for cutting a planar object to be processedcomprising a hexagonal SiC substrate having a main surface forming anangle corresponding to an off-angle with a c-plane, along a line to cut;the method comprising the step of irradiating the object withpulse-oscillated laser light along the line such that a pulse pitchbecomes 10 μm to 18 μm while locating a converging point of the laserlight within the SiC substrate, thereby forming a modified region tobecome a cutting start point within the SiC substrate along the line. 2.A laser processing method according to claim 1, wherein the object isirradiated with the laser light along the line such that the pulse pitchbecomes 12 μm to 14 μm.
 3. A laser processing method according to claim1, wherein the laser light is pulse-oscillated at a pulse width of 20 nsto 100 ns.
 4. A laser processing method according to claim 3, whereinthe laser light is pulse-oscillated at the pulse width of 50 ns to 60ns.
 5. A laser processing method according to claim 1, wherein theobject is cut along the line from the modified region acting as a startpoint after forming the modified region.
 6. A laser processing methodaccording to claim 1, wherein the modified region includes a moltenprocessed region.